1. Field of the Invention
The present invention generally relates to an identification tag and more particularly to an identification tag which can be encoded with multiple bits of information and which can be remotely interrogated and read, and further to a method of producing the tag.
2. Cross-reference to Related Applications
The invention disclosed and claimed herein is related to the inventions disclosed and claimed in co-pending applications Serial Nos. 08/344,805, 08/344,296, 08/344,771 and 08/344,808.
3. Description of the Related Art
For retail tagging, tagging used in the road/air-freight package industry, personnel identification tagging, pallet tagging in manufacturing processes, etc., a tag is required for identifying a product, article or person in detail. With a sufficient number of bits, the tag can be interrogated to determine what the product is, its date of manufacture, its price, whether the product, article or person has been properly passed through a check-out counter or kiosk, etc. Further, identifying the product via a tag can lead to a new type of check-out system for the retail industry giving rise to the much hoped for "no-wait check-out".
Conventional tags and tag systems have had a number of problems including: 1) having only one bit, typical of anti-theft tags, 2) requiring a large amount of power to read the tag, thus requiring a tag battery (or other suitable power source), and 3) being relatively easy to defeat by tampering.
Multibit, remotely-sensed tags are needed for retailing, inventory control and many other purposes. For many applications, the cost must be low and the tags must to be individually encoded. Further, when the tag is interrogated it must produce a distinctive signal to reliably identify the article to which the tag is coupled.
Further, conventional tags have employed the Barkhausen jump effect. However, the Barkhausen jump effect has been used in tags having only a single bit element. Hitherto the invention, tags using the Barkhausen effect have not been transferred to multibit arrays or markers working in concert since this requires a method for personalizing each bit so that it switches at a field without affecting the other bits of the tag. Generally, the Barkhausen effect is characterized by a tendency for magnetization to occur in discrete steps rather than by continuous change, thereby giving rise to a large flux change with time, d.phi./dt, which is key for inducing a sizable voltage in a sensing or pickup coil.
For example, one conventional system uses a large Barkhausen effect from depositions on, for example, a polymer substrate with the system being useful in applications relating to electronic article surveillance and rotation sensors. A number of procedures are available for producing the Barkhausen material including oblique sputtering, heat treatments and generating magnetic fields during and after deposition to obtain maximum flux jumps.
A second layer of hard or semi-hard magnetic material is deposited on the same substrate to provide a means for activating or deactivating the sensor by way of a magnetic field produced by a direct current (DC).
However, this conventional system fails to include an array of Barkhausen elements designed to switch (that is, change its direction of magnetization) as a function of an externally ramped magnetic field with the required field for switching controlled by a third magnetic layer acting as a variable magnetic shunt, differing in thickness for each element.
Further, the conventional system is only for a single bit element using a single Barkhausen layer with no ability to develop a code to distinguish items.
In a second conventional system, a single bit element is employed in which the Barkhausen jump is used to construct an electronic article surveillance tag.
However, this system is deficient in terms of having a three-layer configuration designed to switch at different applied magnetic fields to form a multibit tag. This conventional system uses a second hard magnetic layer deposited on the tag substrate for tag activation/deactivation purposes. Means for producing several identical markers from a single set of deposition/annealing processes are provided, in which a narrow zone is created, from which individual tags can be cut apart without damage. Once separated, the cut-apart tags each serve as individual single bit electronic article surveillance (EAS) markers. However, these markers are not for use in concert or as a multibit array.
Further, this system fails to include means for developing a code by switching at various levels of applied magnetic fields and thus there is no multibit Barkhausen tag for purposes similar to that of the present invention.
Another single-bit conventional tag device includes a hysteresis loop, as shown in FIG. 1A. The active elements of the tag are made of a magnetic material. The tag having the hysteresis loop of FIG. 1A is produced by annealing certain magnetic alloys in a demagnetized (multiple domain) state. Generally, materials (e.g., such as ferromagnetic materials) are divided into magnetic domains, with each domain having a net magnetization even without an external field. A bulk sample will generally not have a net magnetization since the spontaneous magnetization in the various domains will cancel each other due to their random orientations.
The annealing process changes the magnetic anisotropy (e.g., direction of magnetization) so that the domain walls (of the material) become pinned at the positions they had during the annealing process. The walls remain pinned until a predetermined, critical field H.sub.j is exceeded.
At this critical field, the domain walls overcome the barrier to wall displacement and suddenly move a relatively large distance (such as, for example, 100 .mu.m). This wall motion causes a sudden change in magnetization which can be detected in a pickup coil when an applied field exceeds H.sub.j.
FIG. 1A illustrates a hysteresis loop (M vs. H) of a low magnetostriction amorphous alloy after annealing in the demagnetized state for 30 minutes. FIG. 1B illustrates the loop after annealing for one hour. The other significance of these loops is that as shown in FIG. 1B, the major change in magnetization (M) occurs at H.sub.j, the critical field for the Barkhausen jump. In contrast to FIG. 1A, a much smaller discontinuity in M occurs for FIG. 1B so that a more favorable value of d.phi./dt occurs for the hysteresis loop of FIG. 1A, as a result of the longer annealing treatment.
As discussed above, all of the conventional tag devices are disadvantageous in that they are single-bit devices. Further, while a conventional bar code may represent a multibit scheme, a conventional bar code cannot be remotely interrogated and read.